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Coordination of CO to the Alkaline Earth [(MesCshCa]** Peter Selg, Hans H. Brintzinger, * Richard A. Andersen,* and Istvan T. Horvath* Dedicated to Professor Richard R. Schmidt on the occasion of his 60th hirthday

Compared to the countless transition complex­ es, in which CO acts as a 11: acceptor for d electrons on the metal center, CO coordination to main group compounds has been observed in a few instances only. A gas-phase species HCO + has been found to have its v(CO) absorption at 2184 cm~I.[1] Dibo­ rane reacts with CO to give a volatile complex [H3B-CO] with v(CO) = 2165 cm ~ 1.[2] Microwave spectroscopy yields a dipole moment of 1.8 D for this moleculepl consistent with the charge distribution [H3B~ -C=O +].14] Me3Al was found to form a com­ plex [Me3AI-CO] with v(CO) = 2185 cm ~ I in a CO matrix at 15-35 K.[S] The increase of v(CO) above that of free CO (2143 cm ~ 1 )[6] in all of these complexes indicates that CO is acting solely as a a-donor here. In [Me 2 Si-CO] and [(MesCs)2Si-COj,[7. 81 on the other hand, v(CO) is decreased to 1962 and 2065 cm ~ 1, respectively, consistent with a 11:-donor action of these divalent species. Apart from low-temper­ ature matrix studies,[9] CO coordination to alkaline earth com­ pounds has not been reported so far. Here, we describe evidence

[*J Prof. Dr. H. H. Brintzinger. Dipl.-Chem. P. Se]g Fakultat fiir Chemie der Universitat 0-78434 Konstanz (Germany) Telefax: [nt. code + (7531)883137 Prof. Dr. R. A. Andersen Department of Chemistry and Chemical Science Division of Lawrence Berkeley Laboratory University of California Berkeley. CA 94720 (USA) Telefax: Int. code + (510)642-8369 Dr. I. T Horvath Corporate Research Science Laboratories Exxon Research and Engineering Company Annandale. NJ 08801 (USA) Telefax: Int. code +(908)730-3042 [**J This work was supported by the Deutsche Forschungsgemeinschaft. by the Fonds der ChemiscBe Industrie. and by the Director. Office of Energy Re­ search. Office of Basic Energy Sciences. Chemical Sciences Division of the U. S. Department of Energy. under contract DE-AC03-76SF00098. R. A. A thanks the Alexander-van-Humboldt Foundation for a Senior Scientist Award.

791 for the formation of the first carbonyl complex of an organo Table 1. Temperature dependence of equilibrium conSlants K for reaction (a) and alkaline earth molecular compound, [(MesCshCa-COj. K for reaction (b) in toluene, as determined by IR spectroscopy (with e.s.d.).

When a solution of[(MeSCS)2Cajlll] in toluene is exposed to T/eC +32 +10 +3 -9 -18 - 27 CO pressures of 2.5-70 bar in an elevated-pressure IR cell,[12] a new v(CO) absorption appears at 2158 em -I. The appearance Klkbar-! 15(3) 36(8) 51(3) 71(3) 115(14) 127(11) of this band, as well as its loss in intensity upon release of CO K/M-! 2.1 5.1 7.2 10.0 16.1 17.9 pressure, is practically instantaneous; repeated measurements at any given pressure show that these changes are reversible. Figure 1 shows the IR spectrum as a function of CO pressure at 10°C. It is apparent that the change in absorbance at 2158 em - 1, These data are confirmed by a 13C NMR study at elevated CO pressures.[17] Solutions of [(MeSCS)2Caj in [Dsjtoluene un­ ~E(2158), is a suitable measure for the extent of complex forma­ der CO pressures up to 80 bar give only one 13C NMR signal tion. A linear Hildebrand-Benesi plot [13] of ~E(2158) -1 against each for their Cs-ring, CH 3' and CO atoms in the tem­ 18 perature range between + 30 and - 70 °C/ ] indicating that the exchange of CO is rapid on the NMR time scale. The chemical 0.75 shifts of these signals change with increasing CO pressure as indicated in Table 2. As changes in chemical shifts, ~,), are a 0.55

0.35 Table 2. !'C NM R data for [(Me,C ')2CaJ + CO"" [(Me,C ,hCa·COJ in [D8Jtolue· ne at T = 3!.5C [aJ.

0.15 p(CO)/bar b(CH,) b(C,) !i(CO)

-0.05 0 10.39 113.94 184.7 [bJ 15 10.53 113.75 183.36 1 30 10.61 113.62 183.59 0.45 50 10.67 113.55 183.80 80 10.72 113.48 183.96 I. Ig-,- [(C ,Me,),Ca-COJ 10.90(1) [bJ 113.19(4) [bJ 18004 [bJ 0.35 [aJ Measured with a JEOl JMN GX 400 spectrometer operated at 96.4 MHz. chemical shifts relative to b(C"D,CD,) = 2004. [bJ Obtained by least· square analy· 0.25 sis from the values observed at p(CO) = 15-80 bar.

0.15 linear function of the mole fractions of free and complexed 0.05 J b..l&. species, the equilibrium constant K, as well as the limiting values of ~,) for complete complex formation, can be obtained by a -0.05 +---r----,--.----.----., least-squares analysis of the essentially linear plots of ~,) - 1 2250 2200 2150 2100 2050 2000 againstp(CO)-l. A value of K=O.024bar- 1 at 31.5°C, ob­ 1 ,,/em- tained in this manner, is in agreement with that derived from the Fig. 1. IR spectra of a 4 x 10- 2 M solution of [(Me,C,),CaJ in toluene at 10 ec at IR data discussed above. A of,) = 180.4, deter­ CO pressures of 5,10,20,40, and 60 bar; top: Base·line corrected spectra; bottom: mined for CO in [(MeSCS)2Ca-COj, places this CO resonance at Spectra corrected for the absorption of solvent and free CO in solution. higher fields than that of free CO in toluene solution (,) = 184.7). [(MesCshCa-COj differs in this regard from carbonyl complexes with partly filled nd subshells; in these, the p(CO) - I indicates a stoichiometry [(MesCs)2Ca-COjlI41 and an CO have their 13C NMR signals at substantially lower equilibrium constant K = 0.036 bar-I for the reaction between fields than the signal offree CO Y 9, ZOI solvated dec arne thy I calcocene and gaseous CO [Eq. (a)j. Since With regard to both the 13C chemical shift of its CO ligand to higher fields and the shift of v(CO) to higher frequencies, com­ [(MeSCS)2Ca](tol) + CO (gas) ==='" [(MesCs)2Ca-CO](tol) (a) pared to the respective values of free CO, [(MeSCS)2Ca-COj resembles a number of recently reported noble metal CO com­ the concentration of dissolved CO is proportional to CO pres­ plexes, mostly with nd 10 electron configurationY 5J Our results sure, c(CO) = p(CO) x 7.1 X 10-3M bar-I, throughout the show that d electrons are not essential for exothermic CO coor­ temperature and pressure range considered here,[121 a value of dination. [(MeSCS)2Caj appears to make up for its lack of d K = 5.1 M- 1 is obtained for the corresponding reaction involving electrons by an unusual Lewis acidity, which is probably related dissolved CO [Eq. (b)j. IR spectroscopic measurements in the to its bent structureP 1, 22J Metal-ligand bond formation can thus presumably occur with expenditures of reorganization en­ [(MesC sl2Ca](tol) + CO(tol) ==='" [(Me,Cs)2Ca-CO](tol) (b) ergy much smaller than those required to bend typical transition metal sandwich compounds.!22,23J In accord with this notion, temperature range of -27 to + 32 DC give values for the equi­ no carbonyl complex formation is observed, up to CO pressures librium constants as summarized in Table 1 ;[ 161 from these, ther­ of 50 bar, in solutions of [(MesCs)zMgj in toluene, for which a modynamic values of ~H: = - 25 ± 5 kJ mol- 1 and ~S: = strictly linear geometry has been establishedY IJ Preliminary -110 ± 10 J mol- 1 K -1 are obtained for complex formation experiments with the bent alkaline earth metallocene according to Equation (a), while values of ~H~ = [(MeSC5)2Srj [22.24J indicate the formation of a carbonyl com­ - 25 ± 5 kJ mol- 1 and ~S~ = - 70 ± 10 J mol- 1 K - 1 are ob­ plex with v(CO) = 2159 em -1, similar to that observed for tained for Equation (b). [(MeSCS)2Ca-COj.

792 This is the first example of CO binding to a molecular alkaline [20] A 1.1C NMR signal at £l =158 is observed for [H,B-CO]: L. W. Hall. D. W. earth metal compound. Previously described [(MeSCS)2Ca] Lowman. P. D. Ellis. 1. D. Odom. /norg. Chem. 1975. 14. SSO. [21] R. A. Williams. T. P. Hanusa. J. C. Huffman. Orglinomelili/ics 1990. 9, 1128. complexes. such as [(MesCs)zCa(PEt3)].[2S] [(MeSCS)2Ca(Me­ [22] The energy required to change the centroid--centroid angle in the range C=C- Me)].[26] [(MesCs)zCa(Me3Si-C=C-C=C-SiMe3 )].[27] of 180 to 150" has been estimated to he very small: R. Blom. K. Faegri, Jr., and [(MesC 5) 2Ca(CN-2.6-xylyl)2] [25] show that one or two lig­ H. V Volden, Orgal1ollletallic.\·1990. 9. 372: M. Kaupp. P. von R. Schleyer. M. ands can be coordinated at the Ca center. In the case of the Dolg. H. Stoll, JAm. (,hel11. Soc. 1992. 114.8202: T K. Hollis. J. K. Burdett. B. Bosnich. Orgal1olllera/lics 1993. 12. 3385. carbonyl complex reported here. only one CO ligand appears to [23] H. H. Brintzinger. L. L. Lohr. Jr.. K. L. Tang Wong. J Alii. Chclll. Soc. 1975, be taken up in the pressure range studied. The binding of CO to 97.5146: K. M. Simpson. M. F. Rettig. R. M. Wing. Organolllctallics 1992.11. the Ca center is weaker than that of the ligand in 4363. [24] R. A. Andersen. R. Blom. C. 1. Burns. H. V Volden. 1. ChClll. Soc. Cimn. [(Me C 12 Ca(OEt )]' as OEtz is not displaced by CO at pres­ s s z COlllllllln. 1987, 768. sures up to 120 bar. This observation and the similarity of I'(CO) [25] C. 1. Burns. R. A. Andersen. J Organolllel. Ch<'lII. 1987. 3':5. 31. frequencies in [H3B-CO] and [(MeSCS)2Ca-COJ are consistent [26] C. 1. Burns. PhD Thesis. University of California Berkeley. 1987. with the view that CO acts solely as a a-donor ligand towards [27] R A. Williams. T. P. Hanusa. J. C. Huffman. JAm. Chelll. Soc. 1990. J 12. [(MesC S)2Ca] and that dipolar interactions are important for 2454. the Ca-CO hondo

Keywords: compounds· carbonyl complex­ es . high-pressure chemistry·

[1] S. C. Foster. A. R. W McKellar, T. J. Sears, J. Chem. Phrs. 1984.81.578: P. B. Davies. P. A. Hamilton. W J Rothwell, ihid. 1984.81. 1598. [21 A. B. Burg. H. l. Schlessinger. J Am. Chern. Soc. 1937.59. 780: A. B. Burg. ihid. 1952. 74. 3482: G. W Bethke. M. K. Wilson. J. ('hem. Phrs. 1957. 2~. IllS: R. C. Taylor. i/Jid. 1957.26.1131. [3] W Gordy. H. Ring. A. B. Burg. Phl's. Rei'. 1950. 78. 512. [4] M. W P. Strandberg. C. S. Pearsall. M. T. Weiss. J. Chem. Ph),s. 1949,17.429. [51 R. Sanchez. C. Arrington, C. A. Arrington. Jr.. J. Am. Chem. Soc. 1989. 111, 9110. At room temperature no indication of coordination was obtained up to a CO pressure of lObar. [6J K. Nakamoto. b~/"(lred Spectra or Inorganic and Coordination Compounds. 2nd. edition. Wiley. New York. 1970. p. 78. [71 CA. Arrington, J. T. Petty. S. E. Payne. W. C. K. Haskins, JAm. Chem. Soc. 1988. 1111. 6240: M. Pearsall. R. West. ihid. 1988. 110. 7228. [8] M. Tacke. C. Klein, D. J. Stufkens. A. Oskam. P. Jutzi. E. A. Bunte. Z. Anorl(. AI/g. Chelll. 1993,619.865. [9] Coordination of carhon monoxide to the alkaline earth Ouorides MgF,. CaF,. SrI',. and BaF, as well as to CaCl, in CO matrices at temperatures of about 10 K was deduced from the observation of one or several infrared absorption bands he tween 2160 and 2205 cm ~ 1 [lOa] and from changes in the Ca- Cl stretching frequency [lOb]. [10] a) R. H. Hauge. S. E. Gransden. J. L. Margrave. J. ('hem. Soc. Da/lOl1 Trans. 1979. 745: h) I. R. Beattie. P. 1. Jones, N. A. Young. J. Am. Chem. Soc. 1992. J 14.6146. [11] R. A. Andersen, 1. M. Boncella. C. J. Burns, R. 810m. A. Haaland. H. V. Vold­ en. J Orgw/Omel. ('iTetll. 1986. 312. C49: R. A. Andersen. R. Blom. 1. M. Boncella. C. J Burns. H. V. Volden. Acta Chern. S(,([1ll1. Ser. A 1987.41.24. [12] E. U. van Raaij. C. D. Schmulbach, H. H. Brintzinger, J Organomet. Chem. 1987. nt!. 275. Blank experiments in the same cell without [(MeSe),Ca] give rise only to the absorption of free CO at 2134 cm ~ '. [13] H. A. Benesi. 1. H. Hildebrand. J. Am. Chem. Soc. 1949. 71, 2703. [14] A stoichiometry of[(Me,CS),Ca(CO),] cannot be excluded on the basis of the observation of a single CO stretching absorption alone, since the symmetric and asymmetric stretching vibrations of such a dicarbonyl species could be unresolved (ci'. ref. [15]): this stoichiometry is unlikely. however. since a plot of t.E(215R)-1 vs. p(CO)~2 deviates significantly from linearity. [15] ,,-Bonded carbonyl complexes of Pd". Pt2+. Ag+, Au '. Hg2+. and Hgi ' : D. Belli Dell'Amico. F. Calderazzo. P. Robino. A. Segre. J. Choll. Soc. Do/lOll haIlS. 1991. 3017: 1. J. Rack. B. Moasser. J. D. Gargulak. W. L. Gladfelter. H. D. Hochheimer. S. H. Strauss. 1. Chem. Soc. Chem. Commun. 1994.685: H. Willner. M. Bodenhinder. C. Wang, F. Aubke. ihid., 1994. 1189: H. Willner, J Schaehs. G. Hwang. F. Mistry. R. Jones, J Trotter. F. Aubke. JAm. Chem. SoC, 1992. 114.8972: L. Weber. Angelr. Chel11. 1994. 106. 1131; Angelr. Chem. fll/. Ed. Eng/. 1994.33. 1077.

[16] An essentially unchanged value of\'(CO) = 2159 cm ~ 1 and similar absorbance changes in dependence on pressure and temperature are observed in methylcy· clohexane solution and indicate that solvent effects are of minor importance for this cquilihrium reaction. [17] I. T. Horvilth. J. M. Millar. Chem. Rev. 1991. 91. 1339. [18] At lower temperatures. the solubility of [(Me,Cs),Ca] is insufficient for "c NMR measurements. [19] B. E. Mann. B. F. Taylor. 1.1C NMR Data fill' Organometallic Compounds. Acadcmic Press, New York 1981.

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